Subscription Semantics

[gefjon]

Discussion on Discord revealed that there's some confusion among the public about the semantics of subscriptions and how they are processed in the SDK. We should write up a formal-ish spec describing how, when and where subscriptions are processed, with an eye to:

• What work is done by the host vs the SDK.
• What timing and ordering guarantees do and don't exist.

[gefjon]

I wrote some stuff on Discord. This could probably be cleaned up, judiciously edited and reformatted, have examples added, and then slapped on the website.